1. Field of the Invention
The present invention relates to a method for oxide dispersion strengthening of a metallic material with good mechanical property at high temperature, and more particularly, to a method for oxide dispersion strengthening of a metallic material using laser, according to which surface of a metal matrix shaped into sheet or tube is melt by a laser heat source, and an oxide dispersion-strengthening (ODS) powder in nano size is supplied into a site where the melting occurs on the matrix surface, thereby causing homogenous distribution of the ODS powder within the matrix of a target alloy.
2. Description of the Related Art
As industry advances and as a demand for a mechanical part to have higher physical and mechanical properties increases, a method for strengthening an alloy is continuously studied to provide complex of different materials which has endurance to high temperature.
Solid solution strengthening, precipitation strengthening and deformation strengthening are generally used as the alloy strengthening methods. Among these, the solid solution strengthening involves selecting adequate alloying composition and dissolving the composition in the alloy preparation process. The precipitation strengthening either transforms a subject material into a phase with greater strength via heating treatment, or generates specific precipitates. The deformation strengthening or work hardening relates to difficulty of transformation of a material due to increasing dislocation density formed within the material due to deformation. All these strengthening methods have in common that these inhibit movement of dislocation acting in the deformation of the alloy. Because strengthening induces internal change of a material, the reaction mechanism either disappears or results in considerably changed effect depending on temperature of the material.
Accordingly, the oxide dispersion strengthening (ODS), which disperses oxide on a matrix to improve strength and resistance to creep at high temperature, is proposed as a good option. ODS disperses minute oxide particles with good thermal stability on a metallic matrix and provides alloys with greatly increased strength at high temperature than the solid solution strengthening or precipitation strengthening. The relatively higher mechanical strength at high temperature of the oxide dispersion strengthened metal is reported to be attributed to effective controlling on a movement of dislocation by the homogenously dispersed oxide (E. Orown: Trans Inst. Eng. Shipbuild Scotl., 89 (1946)165, F. J. Humphreys and J. W. Martin: Phil. Mag., 16 (1967)927, A. H. Clauer and B. A. Wilcox: Met. Sci. J., 1 (1967)86).
When it comes to conventional art, JP Patent Publication No. 2010-00065302 and US Patent Publication No. 2003-00116239 provide a preparation method of ODS alloy, which includes a mixing step of inputting oxide to a metallic powder as a matrix of an alloy, a mechanical alloying (MA) step of mechanically alloying the mixture, a degassing step, a hot isostatic pressing step, a hot working and cold working step, an annealing step, or the like. However, the above-mentioned preparation method is accompanied with shortcomings such as complicated processing which in turn results in prolonged preparation time, and increasing fabrication cost due to use of expensive metallic powder as a raw material.
Furthermore, U.S. Pat. No. 5,989,491 discloses a preparation method of ODS alloy which adds in advance an oxide forming agent including rare earth material such as zirconium, yttrium, cerium, or lanthanum to a melt alloy. While this preparation method provides homogenous distribution of the oxide forming agent by melting the same along with the matrix alloy, the method also includes the step of pulverization by jetting gas onto the melt alloy. That is, during gas jetting on a melt alloy, argon or nitrogen gas containing maximum 5.0% oxygen is made into alloy powder, alloy powder smaller than a predetermined size is filtered out, and the alloy powder is shaped by roll forging, HIP or hot extrusion. However, while this preparation method can homogenously distribute the oxide forming agent by melting, because powder alloy is prepared by atomization, another melting and shaping is necessary to produce an intended part. As a result, processing becomes complicated.
Korean Publication No. 2010-0127594 discloses a method for preparing ODS alloy efficiently by adding metals targeted for oxidation such as aluminum, titanium, zirconium or yttrium is added during a step of melting matrix metal of the alloy, shaping the mixture, and inputting oxygen during heating treatment of the shaped material to allow oxygen and the metal targeted for oxidation to bind to each other. However, this preparation method has shortcomings of difficulty of propagation of oxygen into the matrix metal and precisely controlling reaction ratio with the metal targeted for oxidation.
As explained above, the conventional ODS alloy preparation methods have shortcoming of different densities between oxide and matrix metal, which in turn results in non-homogenous dispersion, and it takes great time and cost to overcome the above-mentioned shortcomings because it is necessary to conduct mechanical mixture, other various processing such as HIP, and to control the heat treatment to suit the phase change interval. Furthermore, many technical and time limits are posed for the processing to supply oxygen after melting the metal targeted for oxidation with the matrix to control even nano-sized oxide. Furthermore, because the conventional art forms oxide within a bulk-sized matrix metal, strength is already increased in the initial or middle-stage of the production, elongation is decreased and it is difficult to shape the product as desired. Furthermore, special welding technique is necessary to compensate for the deteriorating homogeneity or concentration at a joining area when the prepared parts are joined with each other.
Accordingly, in an effort to find an efficient oxide dispersion strengthening (ODS) method of a metallic material to provide an end product which is economical in terms of both processing and cost, the inventors of the present invention confirmed that an ODS method of distributing oxide particles on a metallic matrix using a laser can provide simpler fabrication process, reduced fabrication cost and efficient production of end products because oxide particles can be supplied directly into a finished sheet or tube, and thus completed the present invention.